Why does Network Render require separate Master and Client roles?
While anyone can make use of network rendering, the main target is for small studios. Which would be the reason this design choice was made.

Imagine a studio with 10 artists, you setup a master and several slaves for the render farm. Each artist is then a client that sends render jobs to the farm, the master keeps track of all these jobs and which slaves are working on each task giving one central point to monitor all work being done by the farm. When the artists go home they can add their machine as a slave so it can keep working through the night. In the morning they can stop it as a slave and get back to their own work.

If each artist was also a master then they would all be fighting to get the slaves to do their work for them.

For a single artist this can be excessive or awkward but is really only a minor issue.